A standard lathe chuck has a spindle that is rotated at high speed and that has a head carrying a plurality of jaws that can be moved radially. The spindle is typically hollow and houses an actuating rod that is axially displaceable in the spindle and that is connected in the head to mechanism that displaces the jaws radially together when the rod is moved axially in one direction and that displaces them axially apart when moved axially oppositely. These jaws can be used to externally or internally grip a workpiece held by the chuck.
It is known to provide on the end of the spindle remote from the chuck head a transmission having an input member rotatable about the spindle axis, an output member axially coupled to the actuating rod, and gearing interconnecting the two members to convert rotation in one direction of the input member into axial movement in one direction of the output member, and opposite rotation into opposite axial movement. The gearing can be at least one shaft extending parallel to the spindle and having one end formed with a pinion meshing with the rotary input member and an opposite end formed with a thread engaged axially in the output member.
German patent document 3,314,629 describes such an arrangement wherein the same motor is used both for moving the jaws during chucking and unchucking of the workpiece and for rotating the chuck during machining. For jaw adjustment the chuck is stopped and both the spindle and the jaw actuator are rotationally arrested and the motor is coupled directly to the input member of the jaw-operating transmission. For subsequent machining the brakes effective on the jaw actuator and on the spindle are released and the motor is reconnected to the spindle so that same and the transmission input element are rotated synchronously.
The main disadvantage of this system is that it is absolutely essential to arrest the chuck during any jaw-adjustment operation. It is impossible to tighten or loosen the jaws while the chuck is rotating, so that valuable time must be lost letting the chuck turn to a halt or to start the chuck rotating each time a workpiece, for instance, is changed. Furthermore tightening the jaws during machining to ensure that the workpiece remains tightly held is impossible.